California at risk of disaster from ‘the Other Big One’, scientists warn

Californians already live with increased risk of wildfires and drought linked to the climate crisis, not to mention the threat of major earthquake on the San Andreas Fault. But now scientists are warning of potential diaster from “the Other Big One”.

New research, led by scientists at University of California, Los Angeles (UCLA) and the National Center for Atmospheric Research, has found that the state faces a catastrophic flood that could inundates vast regions with water flows hundreds of miles long and tens of miles across.

“Every major population center in California would get hit at once — probably parts of Nevada and other adjacent states, too,” said Daniel Swain, UCLA climate scientist and co-author of the paper, in a news release.

About a decade ago, scientists began to examine modern-day flood risk in the state following a major disaster in 1862. During that “Great Flood” – when no flood management practices were in place – waters swamped the state’s Central Valley up to 300 miles long and nearly 60 miles wide.

Around half a million people lived in California back then, compared to 40 million today. A similar event today would leave major cities like Los Angeles and Sacramento under water, even with flood control measures. The damage estimate is $1trillion, the most costly ever.

The findings from the ongoing research project – dubbed “ArkStorm 2.0” to reflect the biblical scale – accounts for how the climate crisis will exacerbate flooding.

“In the future scenario, the storm sequence is bigger in almost every respect,” said Dr Swain. “There’s more rain overall, more intense rainfall on an hourly basis and stronger wind.”

The study predicts that storms later in the century will create 200-400 per cent more run-off in the Sierra Nevada mountain range, which runs through California and up the western US.

The climate crisis is likely to bring more rainfall to California, and increased temperatures means that more rain will fall than snow in the Sierras.

More run-off could lead to devastating landslides and debris flows, particularly in steep areas that have previously been burned by wildfires.

The study, published on Monday in the journal Science Advances, compared two extreme scenarios: one that would occur about once-per-century from the recent historical record, and another in 2081-2100 with projected impacts of the climate crisis.

Both scenarios would involve multiple storms over the course of a month, fueled by atmospheric rivers, so-called “rivers in the sky” which carry heavy rain loads.

The paper also simulated how the storms would affect parts of California at a local level.

“There are localized spots that get over 100 liquid-equivalent inches of water in the month,” Dr Swain said, referring to the future scenario.

“On 10,000-foot peaks, which are still somewhat below freezing even with warming, you get 20-foot-plus snow accumulations. But once you get down to South Lake Tahoe level and lower in elevation, it’s all rain. There would be much more runoff.”

This article is being updated